1. Field of the Invention
The present invention relates generally to a process for preparing multilayer color filters and more particularly to a process for preparing color mosaic filters for use with a flat panel display, such as a passive or active matrix liquid crystal panel, or a display using other light modulating mechanisms in which a variety of color effects is to be produced.
2. Prior Art
Color filters for use with liquid crystal type displays must satisfy the following stringent performance requirements: 1) optical--the filters must exhibit good color, purity and saturation, high transparency, good white balance and the ability to generate a wide range of colors; 2) dimensional--the filters must accurately match display resolution and dimensions and meet rigid registration requirements; 3) flatness--the filters must be very flat to ensure a cell gap of uniform thickness for the liquid crystal; 4) heat resistance--the filters must be capable of withstanding the high temperatures which are encountered during the curing and sealing processes for the liquid crystal displays; 5) color permanence--the filter colors must be stable under prolonged exposure to back lighting and ultraviolet lighting; and 6) chemical resistance--the filters must be stable to withstand the chemical etching and cleaning solutions which are used in processing of the liquid crystal displays.
Various methods for forming color filters or liquid crystal displays are disclosed in the following U.S. Pat. Nos.: No. 4,876,165 to Brewer et al.; No. 4,965,242 to DeBoer et al.; Nos. 4,988,168 and 5,058,997 to Dickerson et al.; No. 4,999,094 to Kamamori et al.; No. 4,953,952 to Okumura et al.; No. 4,870,484 to Sonehara; No. 4,877,697 to Vollmann et al.; No. 4,690,511 to Watanabe; and Nos. 5,042,920 and 5,190,794 to Yoshino et al.
The Dickerson et al. patents are directed to decal technology for forming color filters with crystal display devices. The Brewer et al. patent discloses preparing filters directly on substrates using different colored soluble dies in a polyimide resin which is then formed into suitable patterns by conventional photolithographic techniques. The DeBoer et al. patent discloses a color filter for preparing a color liquid crystal display device by overlaying a die receiving element with a die donor element and heating the die donor element to transfer the die image in a repeating mosaic pattern to the die receiving layer. The Kamamori et al. patent is directed to a method for producing a color filter on a conductive thin film layer having a given set of plural patterns which is formed on an electrically insulated substrate. A colored layer is formed on the conductive thin film layer by electrodepositing a coloring material and a polymer. The Yoshino et al. patents disclose a method for producing a color filter for use with liquid crystal displays wherein the color filter has a plurality of colored layers printed on a transparent substrate with first and second resin layers coated on the coating layers.
All of the above-described methods either cannot meet performance requirements or are very time-consuming and expensive. It is an object of the present invention to produce a color filter for display elements which satisfies all of the performance requirements and which significantly decreases the manufacturing costs of the dyed polyimide process.
The Vollmann et al. patent teaches a color filter constructed from pigmented acrylic layers, and is schematically illustrated in FIG. 1 of the application drawings. The Vollmann et al. patent teaches the lamination of a first pigmented layer 10 onto a first transparent plate 12 which carries an array of display electrodes 14. The first pigmented layer 10 is exposed to actinic radiation followed by the removal of the nonimage layers with a liquid developer. The process is repeated several times with differently colored pigmented layers 16 which forms a color filter stack. However, lamination of the pigmented layers directly onto the display electrodes significantly reduces the productivity of the method because only a single filter can be formed at a given time. The lamination of pigmented layers 10 and 16 in the filter stack with display electrodes 14 below the color filter stack increases the thickness of the gap between display electrodes 14 and electrode 18 formed on the opposite side of liquid crystal layer 20 adjacent an alignment layer 19. Alignment layers 19 are positioned on opposite sides of the liquid crystal layer 20. A transparent plate 12 is adjacent the electrode 18. The increase in the thickness of the gap requires a greater operating voltage. For example, the typical thickness of the liquid crystal layer in a display is 5-6 microns while the thickness of a filter stack, composed of four or more pigmented layers, is at least 25-30 microns, depending on the specific manufacture of the layers. The voltage drop across the filter stack compared to the voltage drop across liquid crystal layer 20 is in approximate proportion to the thickness ratio (i.e., 5:1). Therefore, this arrangement requires the application of approximately six times more voltage than if electrodes 14 and 18 are positioned closely adjacent to liquid crystal layer 20. The increase in the voltage is impractical, requiring higher voltage drive circuitry and correspondingly higher power dissipation. In other applications, for example in thin film electroluminescent displays, the increased voltage becomes even more serious since such displays already require voltage drives on the order of 150 volts. It is an object of the present invention to increase the productivity of the fabrication process, to avoid lamination of the pigmented layers directly onto the display electrodes and to reduce the voltage required by the Vollmann et al. patent.